Given that tag locations cannot be isolated and
?¬?xed, it is important to optimize and ?¬?x the locations of multiple RFID reader antennas so
that the probability of a tag being read is maximized. The readability of a passive tag is
directly related to the amount of power it receives, which in turn, is a function of the
distance between the tag and reader antennas as well as their orientations relative to each
other. Thus the objective of optimal reader antenna placement is equivalent to maximizing
the powering region, that is, the area within the portal where tags can receive the minimal
power required for them to be read at almost all likely orientations of the tag.
10.2 Powering Region with a Single Reader Antenna
Historically, read range has been the primary measure used to measure performance in
RFID applications. In this section, we begin by showing that performance is not only
182 RFID Handbook: Applications, Technology, Security, and Privacy
determined by the relative distance of an RFID tag from a reader, but also by the relative
orientation of the tag??™s antenna with respect to that of the reader. The power received by a
passive tag is determined by Friis??™ equation and includes the effects of orientations and
polarizations. Based on Friis??™ equation, an ef?¬?cient scheme for calculating the powering
region with a single reader antenna is introduced.
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